Johansson et al. U.S. Pat. No. 4,871,509 entitled FUEL COLUMN RETAINER USING RADIALLY COMPRESSED SPRING issued Oct. 3, 1989 disclosed a coil spring for retaining fuel pellets interior of a fuel rod independent of the sealing end plug of the fuel rod. In at least one embodiment, the device included a coil spring having two discrete portions. One locking portion of the coil spring has an outside diameter of the coil spring that slightly exceeds the inside diameter of the fuel rod. This locking portion of the coil spring is used to bear against inside walls of the cladding to radially wedge the spring in place relative to the inside walls of the cladding. Because the locking portion of the spring keys to the inside walls of the cladding, the locking portion of the spring adheres to the cladding even though the end plug of the fuel rod is not yet installed.
Another biasing portion of the spring had a diameter less than the inside diameter of the cladding. This biasing portion of the spring acted from the locking portion to and towards the loaded fuel pellets. This spring portion supplied the necessary bias on the fuel pellets to maintain the pellets in place in the interior of the fuel rod before the ends of the fuel rod cladding were sealed.
Insertion of the locking portion of the spring is easy to understand. Specifically, the locking portion of the coil spring having the larger diameter is literally wound so as to cause the diameter of the coil spring to contract. This locking portion of the spring is thus contracted from a diameter that is slightly larger than the inside diameter of the cladding to a diameter that is slightly smaller than the inside diameter of the cladding. When the spring is suitably contracted in diameter, it is inserted to the designed depth within the cladding tube. Such insertion is made with the biasing portion of the spring being inserted into the cladding first so as to compress the already inserted pellets. Thereafter, the large and spirally wound locking portion of the spring is released. The coil spring unwinds, expands and frictionally keys to the inside walls of the cladding. As a result, inserted fuel pellets are spring biased and held in place after spring insertion and well before sealing of the ends of the cladding. The disclosure also included a tool for the winding insertion of the two portion retainer coil spring.
This two portion locking and biasing coil spring arrangement does have disadvantages. First, the diametrical stress on the cladding by the wound coil spring of the locking portion is additive to that radial stress generated by pressure interior of the fuel rod. This pressure can increase during in-service fuel bundle life. Further, and because the locking spring portion is to and toward the end of the fuel rod cladding, this pressure is concentrated at one end of the fuel rod cladding. More importantly, the amount of this radial force has not been precisely predictable.
Further, the winding of the spring has not always been uniform. Moreover, the locking portion of the spring can become cocked with respect to the cladding. Further, since the coil spring must be constructed from a spring steel such as Inconel having a high neutron capture cross section, the coil spring is not particularly efficient from a nuclear standpoint in its capture of neutrons.
For at least these reasons, the coil spring design has not found wide spread use for pellet retention interior of fuel rods.